Changes between Version 4 and Version 5 of Notes/mmNotes

Timestamp:

09/04/07 15:49:10 (17 years ago)

Author:

VincentMcIntyre

Comment:

add email re 7mm opacity

Notes/mmNotes

@@ -15 +15 @@
    try is at Mopra which has 8GHz of bandwidth and can do dips or paddle calibration (a good summer student project, perhaps?)
 
-   The task to examine is {{{opcor}}. It's unclear how complex the frequency dependency is.
+   The task to examine is {{{opcor}}}. It's unclear how complex the frequency dependency is.
+   {{{
+
+Subject: RE: opacity @7mm
+Date: Thu, 19 Apr 2007 15:06:23 +1000
+From: Bob Sault <rsault@physics.unimelb.edu.au>
+Reply-To: <rsault@physics.unimelb.edu.au>
+To: <Ilana.Feain@CSIRO.AU>
+
+Well ... my guess (without having done any analysis to
+half convince myself!) is that 7mm opacity could
+be handled the same way as 12mm opacity.
+
+At the risk of repeating material you will know
+already - but just to make sure we are talking
+the same language ...
+
+For the 12mm system, you have a system temperature
+measurement from the noise diode. This is a system
+temperature at the feed horn, rather than an above
+atmosphere system temperature. As such, it corrects
+for the effects *except* for opacity. In Miriad
+ATLOD, the opacity is estimated using a model
+atmosphere. The inputs to the model are temperature,
+humidity and pressure. It includes propagation
+effect models which are a function of frequency and
+elevation. It assumes clear skies (no clouds). The model
+will be better than nothing, but it is still just
+a model, and it is trying to simplify a possibly
+complex atmosphere to a few parameters.
+
+However the calibrator and the program source will
+be in the same part of the sky: the opacity difference
+between program source and calibrator should be small
+to start with, and the residual difference should be
+smaller still after the opacity model correction. A
+bigger deal is the difference in the opacity from
+observations of the flux calibrator and the
+secondary calibrator.
+
+My guess is that you can do the same at 7mm as 12mm.
+I assume 7mm has a noise diode??
+
+One problem with sky dips (which I took too long to
+appreciate) is that the antennas are only ~60% efficient.
+That means that effectively 40% of the collecting area is
+looking at other parts of the sky. Because the lower
+elevations in the beam are asymmetrically "hotter" than
+closer to the zenith, this biases sky dips, and they
+do not give the right results. I found, for example,
+that the inferred T_receiver was always poorer when
+the weather was poorer - which implies the deduced
+opacities were on the optimistic side (note T_receiver
+and T_sky should be independent).
+
+--
+   }}}
 
 == 3mm ==
 
+----
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